Electrical and acoustic echo is always present in telephony systems to some degree. Echo becomes a problem when the echo is both loud enough to be perceptible and the echo is delayed relative to the speech source. In the case of local and even most medium distance calls the lag between speech and reception of the echo is in the order of a few milliseconds, and the ear hears this as side tone, which is a normal part of speaking on a telephone handset. When there are appreciable delays, even quite low levels of echo are annoying, and may make speech impossible. Such delays traditionally were due to long transmission and storage paths inherent in intercontinental calls, particularly over satellite links.
Modern packet voice systems such as Voice over IP (VoIP) systems always introduce delays because of processing, packetizing and buffering of the voice signals. Systems that include standard telephony and packetized voice will generally have perceived echo problems unless the level of echo on the line is very low. Thus gateways between traditional telephony systems and packetized systems such as VoIP generally incorporate echo cancellers at the PSTN interface.
As technology has changed, use of echo cancellation in the telephone network at the near end has increased. What was originally used only for intercontinental trunks is now used routinely for all long distance calls exceeding 600 km. Cell phone calls are nearly always echo cancelled and many local calls are echo cancelled by the PSTN because the echo cancellation capacity is there. Calls that include no analog signal anywhere in the circuit such as calls from ISDN telephones or packetized voice calls do not have any electrical echo and therefore usually need no echo cancellation. In practice, therefore, a large fraction of the calls handled by a gateway system are already echo cancelled, and do not need additional echo cancellation.
Echo cancellers are generally special purpose hardware devices. Although the cost of the hardware required to handle every call has come down, it is still significant. It is also possible to perform echo cancellation in software on general purpose devices such as PCs, but echo cancellation consumes significant computing resources which are then not available for other purposes, so that the echo cancellation task limits the practical size of software-based telephony applications such as PBXs or IVRs.
Systems for disabling echo cancellers are known in the prior art. For example, U.S. Pat. No. 6,580,793 issued Jun. 17, 2003 to Dunn et al., discloses a method and apparatus for echo cancellation with self-deactivation. The apparatus of Dunn et al. uses an echo canceller based on an adaptive filter. The adaptive filter determines an estimate of an echo signal present in an untreated signal received from a “near” end of a communications channel and subtracts the estimated echo from the untreated received signal. The difference represents the energy in the estimated echo signal generated by the adaptive filter. If this energy is large, significant echo is present and the echo canceller remains activated. The echo cancellation disabler of Dunn et al. may have disadvantages such as requiring specialized echo cancellers and ties up echo cancellation resources to determine if echo cancellation is required.
Accordingly, a simple, low cost method and system for controlling echo cancellation so as to enable it when required or disable it when it is not required, remains highly desirable.